dtgreene: As an interesting note, I can think of two reasons not to have Aerie (in particular) use the spell.
1. (roleplaying reason) Aerie was trained by Quayle, who specializes in illusion magic. (This can be verified with the Ctrl-Q cheat; he's a gnome cleric/illusionist.) For that reason, she was trained with a focus on illusion, and illusion's opposition school is necromancy. (I would still allow her healing magic, because her class does include cleric.) On one partial playthrough, I used this reasoning as justification for giving her the Illusionist kit; if you do this, be aware that this is overall a net gain in power (+1 mage spell per spell level, can't learn mage necromancy).
Hickory: The BG2 engine does not allow this. If you give an NPC a mage kit (via cheats) they will not be granted the extra spell per level. Even if you grant them this yourself (via cheat), at level up the engine recalculates and the extra spells will be lost. Mods do it by granting permanent 'effects' that give +1 spell per level.
Besides, the are far batter role playing reasons (already stated) for doing so.
Actually, I have done this. In this specific case, Aerie will get the extra mage spells at her next level up. (Not sure if it has to be a mage spell.)
It's the same reason that:
Edwin loses the extra spells per day that he shouldn't have (he apparently comes with *more* than 3 extra spells per level when he should only have 3 extra spells)
Minsk gets the ranger spell he should have had the first time he levels up (in addition to the one for the new level)
Haer-Dalis, if recruited late in Shadows of Amn, gets all the spells per day that he should have after one level up. (Unfortunately, his spellbook is still blank except for spells you've taught him.)